Semiconductor photocatalysts receive light from sunlight or the like and generate photovoltage. An electrochemical reaction is triggered by this photovoltage. Metal oxide semiconductors of titanium oxide (TiO2), strontium titanate (SrTiO2), or the like have been utilized as semiconductor photocatalysts. Titanium oxide is used as an electrode of a photochemical battery. A platinum electrode and a titanium oxide electrode are placed in water. When light is shined on the titanium oxide electrode, electrolysis of water is known to occur. Furthermore, there has been research on photocatalysts in which metals such as platinum are supported by a powder of a metal oxide semiconductor as well as research on electrodes comprising a thin film of titanium oxide on one side of a titanium plate.
When using a titanium oxide photocatalyst for the electrolysis of water by sunlight, only the portion of the spectrum which is approximately 410 nm or lower can be used from the entire light spectrum of sunlight. As a result, the photoelectric conversion efficiency is extremely low. The following are conditions which are necessary for a semiconductor photocatalyst or semiconductor electrode to be able to electrolyze water and for it to be able to adequately utilize the spectrum of sunlight: a photovoltage greater than or equal to the electrolytic voltage of water (theoretical value 1.23 V); a chemical stability so that there is no photodissociation of the semiconductor photocatalyst in the electrolyte solution, and the like.
Because the energy band gap of metal oxide semiconductors of titanium oxide or the like is large, it has the advantages of the electrolysis of water being possible and of not dissolving in the electrolyte solution. However, there is a problem because it does not function as a photocatalyst with the light spectrum when the wavelength is longer than approximately 410 nm. As a result, when conducting chemical reactions using sunlight for the photocatalytic action, only a small portion of the light spectrum of sunlight can be used, and the energy conversion efficiency becomes extremely poor. In order to increase catalytic efficiency, the photocatalyst of titanium oxide or the like is used in the form of a fine powder. However, this flows easily in the electrolyte solution, and as a result, recovery for the purpose of reuse is difficult. With regard to a photocatalyst in which a metal of platinum or the like is supported by a titanium oxide powder, because the anode site (site of oxidation reaction) and the cathode site (site of reduction reaction) exist close to each other, the probability of the reverse reaction is large. This is not very practical.
In U.S. Pat. No. 4,021,323, there is described a technology, wherein: small amounts of molten silicon solution are sprayed from a small nozzle which is placed on the upper end of a shot tower; silicon solution is allowed to free fall, and spherical crystals of silicon are created. However, with this technology, there is the possibility of impurities dissolving into the molten silicon solution from the nozzle. Furthermore, because there is a volume increase when molten silicon solution solidifies, and because solidification begins from the surface, the part which solidifies last will protrude towards the surface of the spherical crystal, and a protruding area is formed. A truly spherical sphere crystal is not formed. However, with the drop tube type experimental apparatus of NASA, because it is equipped with an electromagnetic levitation heating equipment, the material is allowed to melt and free fall.
In this USP, a pn junction is formed on the spherical crystal of silicon. There is also disclosed a solar cell array where there is formed a metal electrode film which is common to a plurality of these sphere crystals (micro photocells). Furthermore, these solar cell arrays are submerged in electrolyte solution. There is also disclosed a photochemical energy conversion device where electrolysis of a solution of hydroiodic acid and hydrobromic acid proceeds by the photovoltage provided by sunlight.
In the silicon solar cell array of this USP, there is not a pair of electrodes formed for each individual micro photocell, but there is a common electrode formed for a plurality of micro photocells. It is not possible to handle individual micro photocells independently. As a result, the micro photocells can not be dispersed in the electrolyte solution as individual semiconductor photocatalysts. Their installation positions can not be changed, nor can they be recovered and reused or washed. The limitations in its use as a semiconductor photocatalyst are extremely large. In addition, in this USP, there is no disclosure regarding the use of semiconductors with photocatalytic capability as electrodes, nor is there disclosure regarding the use of semiconductors which have photocatalytic function and which are selected by considering the reaction activity or reaction selectivity.
Because there is not a pair of electrodes on the surface of each of the microphotocells described above, a single or a plurality of spherical semiconductor elements having a pn junction can not be incorporated into a semiconductor device in such a way that they are independent cells or elements. Because the mode of electrical connection of the plurality of spherical semiconductor elements is fixed, it lacks in generalizability and is not practical.
In the prior art, a color display has been put into practical use. This color display incorporates a plurality of light emitting diodes which are of three types, emitting red light, blue light, or green light. Because each of the light emitting diode lamps can not have a detailed construction, it is not appropriate for small or light weight high resolution displays. In the case of a large size display, the number of parts is large, and it does not have an overall simple construction. The assembling cost is high. Using an integrated circuit technology, it is possible to produce light emitting diode elements which emit 3 colors of light, but the production costs become expensive. The interior integrated circuit becomes complicated, and defective products are more easily generated. This is not practical.
The object of the present invention is to provide the following: a semiconductor device with a spherical semiconductor element which is bead-like, has photovoltaic capability, and has a pair of electrodes; a semiconductor photocatalyst which has excellent photoelectric conversion efficiency and which is practical and generalizable; a semiconductor photocatalyst with an increased electric potential between the oxidizing and reducing electrodes; a semiconductor photocatalyst in which the electric potential between the oxidizing and reducing electrodes can be adjusted freely; a semiconductor device as a solar battery which can receive incident light over a broad range and which has a high utilization efficiency of the semiconductor material; a semiconductor device as a solar battery which has a high degree of freedom in its electrical connections and which has a thin thickness; various semiconductor devices or the like as photodiodes.
A further object of the present invention is to provide the following: a semiconductor device with a spherical semiconductor element which is bead-like, has light emitting capability through a pn junction, and has a pair of electrodes; a semiconductor device as a light emitting element which can emit light over a broad range and which has a high utilization efficiency of the semiconductor material; a semiconductor device as a light emitting element which has a high degree of freedom in its electrical connections and which has a thin thickness; semiconductor devices or the like as light emitting diodes, display panel, or various diodes.